In the fabrication of multi-layer circuit boards and cards, it is necessary to form passages such as vias and through holes from one layer of the structure to a deeper layer. Such passages may be formed mechanically, chemically or by using lasers of various wavelengths to vaporize or burn away the circuit board material. Blind vias are passages that do not pass completely through a board or card. After formation of a blind via, material, such as a metal, may be deposited on the surface of the via to provide an electrical, power, or ground connection between the two layers connected to the via. In other words, the blind vias may be formed for plating of joining metallurgy which provides the interelectrical connections within the multi-layer structure.
Lasers are commonly used to drill blind vias through the insulating polymeric dielectric of multi-layer structures to metal lands to establish interplanar electrical interconnection. These multi-layer structures usually include multiple layers of the same polymeric dielectric material laminated to both sides of the structure; the vias may be formed on either side. In drilling such blind vias, it is critical to the operability of the multi-layer structure that registration between the laser and the internal metal land be achieved. Occurrence of misregistration may result in removal of dielectric material adjacent to the land and continuing to the internal metal planes leading to electrical shorts upon subsequent plating.
Complicating the successful formation of such blind vias and multi-layer structures described above is the fact that successive dielectric layers such as the core and joining level dielectric layer, are often formed of the same dielectric material. While employing the same dielectric material provides certain processing advantages, e.g., being able to use the same hole making techniques for both layers, for example excimer laser drilling, it introduces a problem if misregistration occurs. If misregistration occurs during drilling of blind vias to the underlying lands, this undesirable attack of the inner, core-level dielectric may result. If there is significant misregistration, the laser drilled blind via will not stop at the circuitization layer within the package, but rather, the laser will continue to drill through the structure to the underlying metal layer. Misregistration can be potentially deleterious to the integrity of the package since it can lead to electrical shorts between joining metallurgy and the power plane upon subsequent plating.